1. Field of the Invention
The present invention relates to an apparatus and method for measuring modulation parameters such as a transmitter gain A.sub.0, a droop factor (an amplitude variation per symbol) .sigma..sub.0, a frequency error .OMEGA..sub.0, a phase error .theta..sub.0 and I/Q origin offset B.sub.0 in PSK, DPSK, QAM, GMS and similar quadrature-modulated wave signals modulated by digital data.
2. Description of the Related Art
Conventional apparatuses of this kind are disclosed, for example, in U.S. Pat. Nos. 5,187,719 (issued Feb. 16, 1993) (see the disclosure associated with FIG. 15, in particular) and 5,590,158 (issued Dec. 31, 1996). In these apparatus the input modulated wave signal is converted or transformed into an intermediate frequency signal, which is then converted to a digital signal for measurement of the modulation accuracy of the modulated signal through digital processing. And the intermediate frequency signal converted to digital form is subjected to DFT (Discrete Fourier Transform) processing to determine what is called a clock delay that is a phase difference between a symbol in the modulated wave signal and a symbol clock for measuring the symbol, and the intermediate frequency signal is corrected through digital processing by an interpolation filter in accordance with the clock delay thus determined.
Conventionally, the clock delay is determined through DFT processing of the intermediate frequency signal, so that its sampling rate needs to be sufficiently high, for example, approximately 1 MHz so as to provide the clock delay with required accuracy. Because of such a high sampling rate, much time is spent in digital processing for passing the A/D converted intermediate frequency signal through an intermediate frequency band pass filter and this period of time needed is as long as about one-third the total amount of measuring time of the modulation accuracy measuring apparatus.